High tensile strength steel for container and producing method of the same

ABSTRACT

A steel sheet for containers that has a hardness of 500 MPa or more and superior workability and a method for producing the steel sheet are provided. A steel containing, in percent by mass, 0.01% to 0.05% carbon, 0.04% or less silicon, 0.1% to 1.2% manganese, 0.10% or less sulfur, 0.001% to 0.100% aluminum, 0.10% or less nitrogen, and 0.0020% to 0.100% phosphorus, the balance being iron and incidental impurities, is subjected to hot rolling at a finishing temperature of (Ar 3  transformation temperatute−30)° C. or more and a coiling temperature of 400° C. to 750° C., is subjected to pickling and cold rolling, is subjected to continuous annealing including overaging treatment, and is subjected to second cold rolling at a reduction rate of 20% to 50%, thus providing a high-strength steel sheet for containers that has a tensile strength of 500 MPa or more and a proof stress difference between width and rolling directions of 20 MPa or less.

TECHNICAL FIELD

The present invention relates to high-strength steel sheets for containers suitable as container materials to be subjected to diameter-shape reduction or expansion processing after three-piece processing, such as welding, or two-piece processing, such as DI, and also relates to methods for producing such steel sheets.

BACKGROUND ART

For cost reduction, as well as reductions in material usage and environmental load, product development has recently been proceeding toward a reduction in the product thickness of steel materials (steel sheets) serving as raw materials.

At the same time, because reducing the product thickness decreases rigidity, the strength of the steel materials must be increased to compensate for the decrease in rigidity. An increase in the strength of the steel materials, however, causes a problem in that such hardened materials may be cracked during flange forming or neck forming.

To solve the above problem, various production methods have been devised.

Patent Document 1, for example, proposes a method in which a steel whose constituent contents are controlled within predetermined ranges is hot-rolled to a finishing temperature of (Ar₃ transformation point−30° C.) or more, is subjected to pickling and cold rolling, and is subjected to continuous annealing and secondary cold rolling.

The method of Patent Document 1, however, has a problem in that it is difficult to efficiently, process the steel into thin products, which are therefore not easy to manufacture, and the steel also tends to have a poor appearance, because it contains 0.02% by weight or less phosphorus so that flange formability, neck formability, and corrosion resistance are not degraded and also because the reduction rate in the secondary cold rolling is 15% to 30%. Another problem is that a crack may occur in a surface layer of a slab, thus decreasing a product yield. Furthermore, the steel is difficult to stably produce and needs improvement.

As typical methods for producing hard steel sheets for containers, on the other hand, the following methods are proposed, which are appropriately selected and used depending on the type of annealing (for example, Non-Patent Document 1):

Hot rolling→pickling→cold rolling→box annealing (BAF)→second cold rolling (reduction rate: 20% to 50%)

Hot rolling→pickling→cold rolling→continuous annealing (CAL)→second cold rolling (reduction rate: 20% to 50%)

The above methods, however, have a problem in that a rolled steel sheet has a poor appearance due to uneven density and partial adhesion of rolling oil because a viscous rolling oil is used for improved lubricity during the rolling. In addition, if the rolling reduction rate is high, the steel sheet elongated by rolling has a large proof stress difference between the width and rolling directions thereof.

On the other hand, a method in which the second cold rolling is performed at a lower reduction rate is possible. If the reduction rate is lowered, however, it is difficult to attain the necessary proof stress.

Patent Document 1: Japanese Patent No, 3108615

Non-Patent Document 1: “Wagakuni ni okeru kanyo hyomenshorikoban no gijutsushi (History of Technology for Surface-Treated Steel Sheets for Cans in Japan)”, The Iron and Steel Institute of Japan, issued Oct. 30, 1998, p. 188

Thus, if a steel sheet for containers with a small product thickness is to be produced, no production method satisfactory in terms of all of strength, workability, and productivity is available; such a method is currently demanded.

An object of the present invention, which has been made in light of such circumstances, is to provide a steel sheet for containers that has a tensile strength TS of 500 MPa or more, a proof stress difference between width and rolling directions of 20 MPa or less, and superior workability, and also to provide a method for producing such a steel sheet.

DISCLOSURE OF INVENTION

The inventors made an intensive study to solve the above problems and, as a result, obtained the following findings.

The inventors found that material properties including little tendency for inappropriate appearance, little proof stress difference between width and rolling directions, and high strength can be ensured by controlling the phosphorus content in the constituent composition, performing second cold rolling at a reduction rate of 20% to 50% for increased strength, and performing overaging treatment during continuous annealing for homogeneous precipitation of carbide so that it serves as a site for dispersing strain during processing. In addition, the inventors found that a steel sheet for containers with further improved workability can be provided by specifying the grain size, density, and proportion of the carbide.

In the present invention, as described above, a high-strength steel sheet for cans has been completed by controlling the constituent contents thereof on the basis of the above findings.

The present invention, based on the above findings, is summarized as follows.

[1] A high-strength steel sheet for containers, containing, in percent by mass, 0.01% to 0.05% carbon, 0.04% or less silicon, 0.1% to 1.2% manganese, 0.10% or less sulfur, 0.001% to 0.100% aluminum, 0.10% or less nitrogen, and 0.0020% to 0.100% phosphorus, the balance being iron and incidental impurities, the steel sheet having a tensile strength TS of 500 MPa or more and a proof stress difference between width and rolling directions of 20 MPa or less.

[2] A high-strength steel sheet for containers, containing, in percent by mass, 0.01% to 0.05% carbon, 0.04% or less silicon, 0.1% to 1.2% manganese, 0.10% or less sulfur, 0.001% to 0.100% aluminum, 0.10% or less nitrogen, and 0.0020% to 0.020% phosphorus, the balance being iron and incidental impurities, the steel sheet having a tensile strength TS of 500 MPa or more and a proof stress difference between width and rolling directions of 20 MPa or less.

[3] A method for producing a high-strength steel sheet for containers, wherein a steel containing, in percent by mass, 0.01% to 0.05% carbon, 0.04% or less silicon, 0.1% to 1.2% manganese, 0.10% or less sulfur, 0.001% to 0.100% aluminum, 0.10% or less nitrogen, and 0.0020% to 0.100% phosphorus, the balance being iron and incidental impurities, is subjected to hot rolling at a finishing temperature of (Ar₃ transformation temperature−30)° C. or more and a coiling temperature of 400° C. to 750° C., is subjected to pickling and cold rolling, is subjected to continuous annealing including averaging treatment, and is subjected to second cold rolling at a reduction rate of 20% to 50%.

[4] A method for producing a high-strength steel sheet for containers, wherein a steel containing, in percent by mass, 0.01% to 0.05% carbon, 0.04% or less silicon, 0.1% to 1.2% manganese, 0.10% or less sulfur, 0.001% to 0.100% aluminum, 0.10% or less nitrogen, and 0.0020% to 0.020% phosphorus, the balance being iron and incidental impurities, is subjected to hot rolling at a finishing temperature of (Ar₃ transformation temperature−30)° C. or more and a coiling temperature of 400° C. to 750° C., is subjected to pickling and cold rolling, is subjected to continuous annealing including overaging treatment, and is subjected to second cold rolling at a reduction rate of 20% to 50%.

In the present description, all percentages indicating the contents of steel constituents are expressed by mass. In the present invention, additionally, the term “high-strength steel sheet for containers” refers to a steel sheet for containers that has a tensile strength TS (hereinafter also simply referred to as TS) of 500 MPa or more.

In addition, the high-strength steel sheets of the present invention for containers are intended as container materials and can materials. The steel sheets, irrespective of whether or not they are surface-treated, are tin-plated, nickel-tin-plated, or chromium-plated (so-called tin-free plating), or are further coated with, for example, an organic material, so that they can be used for a vast range of applications.

In addition, although not specifically limited, the thickness is preferably 0.30 mm or less, more preferably 0.20 mm or less, in view of maximizing utilization of the present invention and achieving the best result. In particular, a thickness of 0.170 mm or less is preferred.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in detail.

A steel sheet of the present invention for containers is a high-strength steel sheet for containers that has a TS of 500 MPa or more and a proof stress difference between width and rolling directions of 20 MPa or less. In the present invention, such a high-strength steel sheet for containers can be provided by controlling the phosphorus content and performing second cold rolling (hereinafter also referred to as secondary cold rolling) at a reduction rate of 20% to 50%.

The constituent composition of the steel sheet of the present invention for containers will be described.

Carbon: 0.01% to 0.05%

If the carbon content is high, it unnecessarily hardens the steel sheet after the secondary cold rolling and degrades can formability and neck formability. This element also contributes to HAZ cracking due to noticeable hardening at a weld during flange forming. The carbon content is controlled to 0.05% or less because if it exceeds 0.05%, the above effects appear noticeably. On the other hand, the carbon content is controlled to 0.01% or more because if it is extremely low, the secondary cold rolling must be performed at a high reduction rate to maintain container strength. Preferably, the carbon content is controlled to 0.02% to 0.04%, more preferably 0.02% to 0.03%.

Silicon: 0.04% or Less

Adding a large amount of silicon degrades, for example, surface condition and corrosion resistance. Hence, the silicon content is controlled to 0.04% or less.

Manganese: 0.1% to 1.2%

Manganese is an element effective in preventing a hot crack due to sulfur; adding manganese depending on the amount of sulfur provides the effect of preventing a crack. In addition, manganese has the effect of reducing the size of crystal grains. To achieve the above effects, manganese must be added in an amount of at least 0.1% or more. On the other hand, the upper limit is set to 1.2% because adding a large amount of manganese tends to degrade corrosion resistance and also unnecessarily hardens the steel sheet and degrades flange formability and neck formability. Preferably, the manganese content is controlled to 0.35% or less.

Phosphorus: 0.0020% to 0.100%

Phosphorus, a constituent hardening the steel, is contained in a predetermined amount depending on the desired strength in the present invention. The phosphorus content is controlled to 0.0020% or more because if it falls below 0.0020%, a TS of 500 MPa or more cannot be achieved. On the other hand, an excessive phosphorus content degrades corrosion resistance and also degrades flange formability and neck formability. The upper limit is set to 0.100% because if the phosphorus content exceeds 0.100%, the above effects appear noticeably. Preferably, the phosphorus content is controlled to 0.0020% to 0.020% because a higher strength can be achieved by the addition of phosphorus, which provides appropriate strength, and the effect of the secondary cold rolling, described later.

Sulfur: 0.10% or Less

Sulfur, present as inclusions in the steel, is an element that decreases the ductility of the steel sheet and degrades its corrosion resistance. Hence, the sulfur content is controlled to 0.10% or less. Preferably, the sulfur content is 0.030% or less.

Aluminum: 0.001% to 0.100%

Aluminum is an element necessary for deoxidation of the steel. If the aluminum content falls below 0.001%, the deoxidation proceeds insufficiently, and inclusions degrade flange formability and neck formability. Hence, the aluminum content is controlled to 0.001% or more. On the other hand, aluminum combines with nitrogen to decrease the content of dissolved nitrogen; if the content of dissolved nitrogen is extremely decreased, the necessary strength cannot be achieved. Hence, the aluminum content is controlled to 0.100% or less. Preferably, the aluminum content is controlled to 0.035% to 0.075%.

Nitrogen: 0.10% or Less

Nitrogen is an element useful for increasing strength without increasing hardness at a weld. If the nitrogen content is excessive, however, it noticeably hardens the steel sheet and significantly increases the risk of a crack defect in a rolling material (slab), thus degrading flange formability and neck formability. Hence, the nitrogen content is controlled to 0.10% or less. Preferably, the nitrogen content is controlled to 0.05% or less. In view of preventing slab cracking, the nitrogen content is more preferably controlled to less than 0.01%, most preferably 0.005% or less. Thus, a reduced nitrogen content decreases the risk of slab cracking and improves yield without the need for maintenance of a slab.

Balance: Iron and Incidental Impurities

The balance other than the above constituents is iron and incidental impurities. As incidental impurities, for example, 0.01% or less tin is permissible.

The steel sheet of the present invention for containers, having the above composition, has a TS of 500 MPa or more and a proof stress difference between width and rolling directions of 20 MPa or less. If the steel sheet has a TS of 500 MPa or more, the rigidity thereof is not decreased even if the thickness is reduced. In addition, because the proof stress difference between width and rolling directions is 20 MPa or less, no crack occurs during flange forming or neck forming.

Next, a method for producing the high-strength steel sheet of the present invention for containers will be described.

A molten steel having the above composition is prepared by a typical known preparation method using, for example, a converter and is cast into rolling materials (slabs) by a typical known casting method such as continuous casting. These rolling materials are then subjected to hot rolling to form hot-rolled sheets.

Slab Removal Temperature: 1,050° C. to 1,300° C. (Preferred Condition)

If the slab removal temperature is 1,050° C. or more, a sufficiently high hot-rolling finishing temperature can be ensured in the subsequent hot-rolling step. On the other hand, if the removal temperature is 1,300° C. or less, the surface condition of the steel sheet is not ultimately degraded. Hence, the slab removal temperature is preferably 1,050° C. to 1,300° C.

Finishing Temperature (Hot-Rolling Finishing Temperature): (Ar₃ Transformation Temperature−30)° C. or More

The hot-rolling finishing temperature must be controlled to (Ar₃ transformation point−30)° C. or more to achieve ease of cold rolling in the subsequent step and superior product properties. If the hot-rolling finishing temperature falls below (Ar₃ transformation point−30)° C., coarse grains are formed in the metallographic structure of the final product, thus tending to cause surface roughness during can forming. In addition, a rigging phenomenon occurs at a low hot rolling finishing temperature, thus tending to result in a poor appearance after forming. Hence, the hot-rolling finishing temperature is controlled to (Ar₃ transformation point−30)° C. or more.

Coiling Temperature: 400° C. to 750° C.

The coiling temperature is controlled to 400° C. or more because if it is extremely low, the hot-rolled sheet is degraded in shape and has a problem in the operation in the subsequent pickling and cold-rolling step. On the other hand, if the coiling temperature is extremely high, a sufficient amount of dissolved nitrogen for strengthening cannot be ensured because aluminum nitride precipitates in the hot-rolling mother plate. In addition, a carbide-aggregated structure is formed in the hot-rolling mother plate and impairs the effect of homogeneously precipitating carbide by averaging, described later, and this in turn adversely affects the corrosion resistance of the steel sheet. Furthermore, a thickened scale on the surface of the steel sheet decreases ease of pickling. To avoid such problems, the coiling temperature must be controlled to 750° C. or less.

The hot-rolled sheet thus produced is subjected to pickling and cold rolling to form a cold-rolled sheet. The pickling may be performed to remove surface scale by a typical method using an acid such as hydrochloric acid or sulfuric acid.

Reduction Rate in Cold Rolling (After Pickling): 80% or More (Preferred Condition)

The reduction rate is preferably 80% or more because if it falls below 80%, a structure with insufficiently fine grains may be formed after annealing. For a steel sheet of such a material as used in the present invention, the reduction rate is more preferably 85% or more to achieve a structure with sufficiently fine grains. On the other hand, there is no need to set the upper limit of the reduction rate; it is optionally set in terms of, for example, the capability of the equipment for hot rolling and cold rolling.

Annealing Temperature: 800° C. or Less within Recrystallization Temperature Range (Preferred Condition)

If an unrecrystallized structure remains in the steel sheet, it must be subjected to recrystallization treatment by continuous annealing because the structure results in, for example, poor formability in can forming and a poor appearance. If the annealing temperature is extremely high, however, defects such as heat buckles and sheet fractures occur during continuous annealing. In addition, the risk of degraded appearance properties due to abnormal growth of crystal grains is increased. Hence, the annealing temperature is preferably 800° C. or less within a recrystallization temperature range.

In addition, within this temperature range, the steel sheet does not have to be maintained at constant temperature. In view of stability of operation, an equivalent soaking time of 5 to 60 seconds is sufficient. A soaking time of 5 seconds or more is preferred because a sufficient amount of carbide precipitates, which serves as a site for dispersing stress during processing.

Overaging Treatment

Overaging treatment is required to disperse the carbide precipitated by the annealing more homogeneously so that it serves effectively as a stress-dispersing site. The overaging treatment is preferably performed by cooling the steel sheet to a temperature range of 300° C. to 500° C. at a cooling rate of 10° C/s or more after the annealing and maintaining it in a temperature range of 300° C. to 500° C. for 5 seconds or more. Cooling the steel sheet to a temperature range of 300° C. to 500° C. at a cooling rate of 10° C./s or more allows more carbide to precipitate, whereas maintaining it in a temperature range of 300° C. to 500° C. for 5 seconds or more ensures homogeneous precipitation of carbide. In addition, after the overaging treatment, the second cold rolling, described below, can be performed at a reduction rate of 20% to 50% with a proof stress difference between width and rolling directions of 20 MPa or less. If the overaging treatment is performed under such conditions, the densities and proportions of carbide grains with a size of not more than 1.5 μm and with a size of more than 1.5 μm and not more than 3.0 μm can be controlled within preferred ranges described later.

Reduction Rate in Second Cold Rolling: 20% to 50% (Preferably, 20% to 30%)

The second cold rolling (hereinafter also referred to as secondary cold rolling) after the continuous annealing is required to ensure sufficient pressure strength for welded cans, that is, sufficient yield strength for the steel sheet. In particular, for the material of the present invention whose phosphorus content is controlled, a reduction rate of at least 20% is required in the secondary cold rolling. If the reduction rate exceeds 50%, on the other hand, the material properties become highly anisotropic, and the proof stress difference between width and rolling (rolling) directions exceeds 20 MPa. In addition, flange formability and neck formability are noticeably degraded in the new cutting method (cutting method in which the rolling direction of a steel sheet is parallel to the axial direction of a can body). Furthermore, the amount of strain released is increased after welding during can forming, and softening occurs noticeably in a welding-heat-affected zone, thus tending to cause flange cracking. Hence, the reduction rate is controlled to 50% or less. Preferably, the reduction rate is controlled to 20% to 30%, although it may be appropriately selected depending on the phosphorus content and the desired strength of the steel sheet. Specifically, a relatively low reduction rate is preferred if the phosphorus content is high, namely, more than 0.020% and not more than 0.100%.

In the present invention, a plating layer can be formed on a surface (at least one surface) of the cold-rolled steel sheet after the secondary cold rolling to form a plated steel sheet. The plating layer formed on the surface may be of any type applied to steel sheets for containers. The plating layer can be exemplified by tin plating, chromium plating, nickel plating, and nickel-chromium plating. In addition, no problem arises if, for example, painting or laminating an organic resin film is performed after the plating treatment.

EXAMPLES

Steels containing the constituents shown in Table 1 with the balance being iron and incidental impurities were prepared by a converter and were cast into slabs by continuous casting. The slabs were then subjected to hot rolling at a slab removal temperature of 1,200° C., a hot-rolling finishing temperature of 900° C., and a coiling temperature of 650° C. to form hot-rolled sheets with a finish thickness of 2.0 mm. Subsequently, the hot-rolled sheets were subjected to descaling treatment by pickling and then cold rolling at a reduction rate of 90% to form cold-rolled sheets with a finish thickness of 0.20 mm, and they were subjected to continuous annealing at a soaking temperature of 750° C. for a soaking time of 10 to 30 seconds and were then subjected to overaging treatment and secondary cold rolling to produce cold-rolled steel sheets.

The conditions for the overaging treatment and the reduction rate in the secondary cold rolling are as shown in Tables 2 and 3.

TABLE 1 Ar₃ trans- Steel Steel constituents (percent by mass) formation symbol C Si Mn P S Al N point (° C.) A 0.02 0.015 0.15 0.011 0.004 0.005 0.0100 890 B 0.02 0.015 0.15 0.020 0.004 0.005 0.0100 890 C 0.02 0.015 0.15 0.021 0.004 0.005 0.0065 890 D 0.02 0.015 0.15 0.019 0.004 0.005 0.0043 890

The structures of the steel sheets thus produced were observed to determine the densities and proportions of carbide grains for different sizes by the following method, and the properties thereof were evaluated by the following tests.

The cold-rolled steel sheets produced as described above were embedded in bakelite resin, and sections thereof were polished. The steel sheets were then immersed in a corrosive solution, namely, a sodium picrate solution prepared by mixing picric acid and sodium hydroxide, at 80° C. for 60 seconds. Carbide was then observed in three fields of view (regions with a size of about 0.1375 mm×0.1375 mm) using an optical microscope at a magnification of 400 times. In each field of view, the numbers of carbide grains with a size of not more than 1.5 μm, with a size of more than 1.5 μm and not more than 3.0 μm, and with a size of more than 3.0 μm were visually counted, and the average density and the average proportion in the three fields of view were determined. The size of a carbide grain herein refers to the minimum size thereof; for example, if a carbide grain has a shape with minor and major axes, such as a rectangular or elliptical shape, the minimum size thereof is used as the grain size in the present invention.

(i) Tensile Test

JIS No. 13-B tensile test pieces were sampled in the rolling (L) direction from the centers of the cold-rolled steel sheets in the width direction and were subjected to a tensile test at a strain rate crosshead rate of 10 mm/s to measure the tensile strengths TS and yield strengths YS thereof. The tensile test was carried out within a day after the production. JIS No. 13-B test pieces were used as the tensile test pieces to minimize a fracture phenomenon outside gauge marks.

(ii) Proof Stress Difference Between Width and Rolling Directions

The difference between the YS measured by the tensile test in Item (i) above and the YS, measured as in Item (i), of JIS No. 13-B tensile test pieces sampled in the width direction was determined.

(iii) Neck Formability

The cold-rolled steel sheets were tin-plated (amount of tin deposited per surface: 2.8 g/m²) to form plated steel sheets. After the surfaces of the plated steel sheets were subjected to painting, printing, and transparent varnish finishing, the steel sheets were subjected to deep drawing including cup drawing and double redrawing a hundred times without using press oil under the following conditions to examine the incidence of drawing wrinkles at necks:

-   Deep drawing conditions -   Blank diameter: 200 mm -   Lubrication condition: no press oil used -   Drawing ratio of first drawing: 1.5 -   Drawing ratio of second drawing: 1.2 -   Drawing ratio of third drawing: 1.2 -   Blank holder force in first to third drawing: optimum condition -   Flange forming: 8% elongation -   Radius of redrawing die shoulder: 0.45 mm -   Processing speed: 0.3 m/s

(iv) Resistance to Flange Cracking

The incidence of flange cracking was examined in the deep drawing of Item (iii).

(v) Appearance

The cold-rolled steel sheets were visually observed, and portions judged as being different in luster or color were determined to have a poor appearance. Among observation units of 100 m, a particular unit was determined to be a portion with a poor appearance if at least one site with a poor appearance was found in that unit. The poor appearance rate was determined by observation over a length of 10,000 m.

(vi) Slab Cracking

The surfaces of the slabs after the continuous casting were visually observed for slab cracking. Among observation units of 1 m, a particular unit was determined to be a portion with a poor appearance if at least one crack was found in that unit. The poor appearance rate was determined by observation over a length of 10 m.

The obtained results are shown in Tables 2 and 3 along with the conditions.

TABLE 2 Density of carbide (grains/10,000 μm²) Grain size Secondary Grain of more Overaging treatment cold rolling size of than 1.5 μm Steel Cooling Retention Reduction not more and not Grain size of sheet Steel rate Temperature time rate than 1.5 more than more than No. Category symbol (° C./s) (° C.) (s) (%) μm than 3.0 μm 3.0 μm  1 Comparative A 40 — 0 10  90 75 30 example  2 Comparative A 40 — 0 15  90 75 30 example  3 Comparative A 40 — 0 20  90 75 30 example  4 Comparative A 40 — 0 25  90 75 30 example  5 Comparative A 40 — 0 30  90 75 30 example  6 Comparative A 40 500 5 10 105 75 20 example  7 Comparative A 40 500 5 15 105 75 20 example  8 Invention A 40 500 5 20 105 75 20 example  9 Invention A 40 500 5 30 105 75 20 example 10 Invention A 40 500 5 50 105 75 20 example 11 Comparative B 40 500 5 10 108 70 27 example 12 Comparative B 40 500 5 15 108 70 27 example 13 Invention B 40 500 5 20 103 64 20 example 14 Invention B 40 500 5 30 103 64 20 example 15 Invention B 40 500 5 50 103 64 20 example 16 Invention B 25 500 5 20  85 85 35 example 17 Invention B 30 500 5 20  90 75 32 example 18 Invention B 35 500 5 20  95 68 28 example Steel sheet properties and evaluation results Proof stress Proportion in all carbide difference Grain size Grain size Tensile between width Incidence Incidence Poor Incidence Steel of not more of not more strength and rolling of neck of flange appearance of slab sheet than 1 .5 μm than 3.0 μm TS directions wrinkles cracking rate cracking No. (%) (%) (MPa) (MPa) (%) (%) (%) (%)  1 46 85 450 10 0 0 0 5  2 46 85 490 10 0 0 0 5  3 46 85 550 25 5 10 0 5  4 46 85 600 30 7 15 0 5  5 46 85 650 30 10 20 5 5  6 53 90 430 0 0 0 0 5  7 53 90 470 0 0 0 0 5  8 53 90 530 5 0 0 0 5  9 53 90 570 5 0 0 0 5 10 53 90 620 10 2 1 1 5 11 53 87 450 0 0 0 0 5 12 53 87 490 0 0 0 0 5 13 55 89 550 3 0 0 0 5 14 55 89 590 3 0 0 0 5 15 55 89 640 9 2 0 1 5 16 41 83 550 3 5 3 0 5 17 46 84 550 5 3 3 0 5 18 50 85 550 7 2 2 0 5

TABLE 3 Density of carbide (grains/10,000 μm²) Grain size Secondary Grain of more Overaging treatment cold rolling size of than 1.5 μm Steel Cooling Retention Reduction not more and not Grain size of sheet Steel rate Temperature time rate than 1.5 more than more than No. Category symbol (° C./s) (° C.) (s) (%) μm 3.0 μm 3.0 μm 19 Comparative C 40 — 0 10  90 75 30 example 20 Comparative C 40 — 0 15  90 75 30 example 21 Comparative C 40 — 0 20  90 75 30 example 22 Comparative C 40 — 0 25  90 75 30 example 23 Comparative C 40 — 0 30  90 75 30 example 24 Comparative C 40 500 5 10 105 75 20 example 25 Comparative C 40 500 5 15 105 75 20 example 26 Invention C 40 500 5 20 105 75 20 example 27 Invention C 40 500 5 30 105 75 20 example 28 Invention C 40 500 5 50 105 75 20 example 29 Comparative D 40 500 5 10 108 70 27 example 30 Comparative D 40 500 5 15 108 70 27 example 31 Invention D 40 500 5 20 103 64 20 example 32 Invention D 40 500 5 30 103 64 20 example 33 Invention D 40 500 5 50 103 64 20 example 34 Invention D 25 500 5 20  85 85 35 example 35 Invention D 30 500 5 20  90 75 32 example 36 Invention D 35 500 5 20  95 68 28 example Steel sheet properties and evaluation results Proportion in all carbide Proof stress Grain Grain difference size of size of Tensile between width Incidence Incidence Poor Incidence Steel not more not more strength and rolling of neck of flange appearance of slab sheet than 1.5 μm than 3.0 μm TS directions wrinkles cracking rate cracking No. (%) (%) (MPa) (MPa) (%) (%) (%) (%) 19 46 85 450 10 0 0 0 0 20 46 85 490 10 0 0 0 0 21 46 85 550 25 5 10 0 0 22 46 85 600 30 7 15 0 0 23 46 85 650 30 10 20 5 0 24 53 90 430 0 0 0 0 0 25 53 90 470 0 0 0 0 0 26 53 90 530 5 0 0 0 0 27 53 90 570 5 0 0 0 0 28 53 90 620 10 2 1 1 0 29 53 87 450 0 0 0 0 0 30 53 87 490 0 0 0 0 0 31 55 89 550 3 0 0 0 0 32 55 89 590 3 0 0 0 0 33 55 89 640 9 2 0 1 0 34 41 83 550 3 5 3 0 0 35 46 84 550 5 3 3 0 0 36 50 85 550 7 2 2 0 0

According to Tables 2 and 3, the invention examples, namely, Nos. 8 to 10, 13 to 18, 26 to 28, and 31 to 36 had sufficient strengths and proof stress differences between width and rolling directions of not more than 20 MPa, thus sufficiently achieving, for example, the performance required for three-piece processing. In addition, it was found that the invention examples had a superior appearance without neck wrinkles or flange cracking. In particular, for Nos. 8 to 10, 13 to 15, 26 to 28, and 31 to 33, in which the densities and proportions of carbide fell within preferred ranges, it was found that they had further improved workability.

On the other hand, Nos. 1, 2, 19, and 20, which are comparative examples in which no overaging treatment was performed, had insufficient strengths because the reduction rate in the secondary cold rolling was low. Although Nos. 3 to 5 and 21 to 23 had high strengths because the reduction rate in the secondary cold rolling was not less than 20%, the proof stress difference between L and C directions exceeded 20 MPa, and neck wrinkles and flange cracking occurred noticeably. In addition, a poor appearance resulted.

In addition, Nos. 6, 7, 11, 12, 24, 25, 29, and 30 had insufficient strengths because the reduction rate in the secondary cold rolling fell below 20%.

Furthermore, the following findings concerning the density and proportion of carbide were obtained. Preferably, in terms of the workability of the high-strength steel sheet of the present invention for containers, the density of carbide grains with a size of not more than 1.5 μm exceeds 102 grains/10,000 μm², and the density of carbide grains with a size of more than 1.5 μm and not more than 3.0 μm exceeds 63 grains/10,000 μm². In addition, more preferably, the proportion of the number of carbide grains with a size of not more than 1.5 μm to the total number of carbide grains exceeds 52%, and the proportion of the number of carbide grains with a size of not more than 3.0 μm to the total number of carbide grains exceeds 85%.

If the density of carbide grains with a size of not more than 1.5 μm exceeds 102 grains/10,000 μm² and the density of carbide grains with a size of more than 1.5 μm and not more than 3.0 μm exceeds 63 grains/10,000 μm², a sufficient amount of carbide, which functions as a stress-dispersing site during processing, can be ensured, so that the workability is further improved. More preferably, the density of carbide grains with a size of not more than 1.5 μm is 130 grains/10,000 μm² or more and the density of carbide grains with a size of more than 1.5 μm and not more than 3.0 μm is 80 grains/10,000 μm² or more.

In addition, if the proportion of the number of carbide grains with a size of not more than 1.5 μm to the total number of carbide grains exceeds 52% and the proportion of the number of carbide grains with a size of not more than 3.0 μm to the total number of carbide grains exceeds 85%, the effect of carbide, which functions as a stress-dispersing site, is further enhanced, so that the workability is further improved. More preferably, the proportion of the number of carbide grains with a size of not more than 1.5 μm to the total number of carbide grains is 55% or more, and the proportion of the number of carbide grains with a size of not more than 3.0 μm to the total number of carbide grains is 90% or more.

In addition, the density and proportion of carbide can be controlled by annealing a cold-rolled steel sheet under predetermined conditions. Specifically, the averaging treatment is performed in the continuous annealing step after cold rolling while controlling the heat history of the steel sheet within a predetermined range.

Furthermore, Table 3 shows examples in which the nitrogen content was 0.0065% or 0.0043%, which fell within a preferred range, namely, less than 0.01%. According to Table 3, no slab cracking was found if the nitrogen content fell below 0.01%; thus, it was found that slab cracking was prevented.

According to the present invention, a high-strength steel sheet for containers that has a TS of 500 MPa or more, a proof stress difference between width and rolling directions of 20 MPa or less, and superior workability with which no crack occurs during flange forming or neck forming is provided.

In the present invention, additionally, high strength is achieved by controlling the phosphorus content and setting the reduction rate in the second cold rolling to 20% to 50%, and the problems of the appearance after rolling and the proof stress difference between width and rolling directions are also solved.

In addition, slab cracking can be prevented by controlling the nitrogen content within a preferred range, namely, less than 0.01%, thus reducing a decrease in product yield.

INDUSTRIAL APPLICABILITY

The steel sheet of the present invention for containers has superior strength without being cracked during neck forming or flange forming and is therefore suitable for use as a container material, for example, for food containers such as cans, nonfood containers such as oil filters, and electronic parts such as batteries. 

1. A high-strength steel sheet for containers, containing, in percent by mass, 0.01% to 0.05% carbon, 0.04% or less silicon, 0.1% to 1.2% manganese, 0.10% or less sulfur, 0.001% to 0.100% aluminum, 0.10% or less nitrogen, and 0.0020% to 0.100% phosphorus, the balance being iron and incidental impurities, the steel sheet having a tensile strength TS of 500 MPa or more and a proof stress difference between width and rolling directions of 20 MPa or less.
 2. A high-strength steel sheet for containers, containing, in percent by mass, 0.01% to 0.05% carbon, 0.04% or less silicon, 0.1% to 1.2% manganese, 0.10% or less sulfur, 0.001% to 0.100% aluminum, 0.10% or less nitrogen, and 0.0020% to 0.020% phosphorus, the balance being iron and incidental impurities, the steel sheet having a tensile strength TS of 500 MPa or more and a proof stress difference between width and rolling directions of 20 MPa or less.
 3. A method for producing a high-strength steel sheet for containers, wherein a steel containing, in percent by mass, 0.01% to 0.05% carbon, 0.04% or less silicon, 0.1% to 1.2% manganese, 0.10% or less sulfur, 0.001% to 0.100% aluminum, 0.10% or less nitrogen, and 0.0020% to 0.100% phosphorus, the balance being iron and incidental impurities, is subjected to hot roiling at a finishing temperature of (Ar₃ transformation temperature−30)° C. or more and a coiling temperature of 400° C. to 750° C., is subjected to pickling and cold rolling, is subjected to continuous annealing including overaging treatment, and is subjected to second cold rolling at a reduction rate of 20% to 50%.
 4. A method for producing a high-strength steel sheet for containers, wherein a steel containing, in percent by mass, 0.01% to 0.05% carbon, 0.04% or less silicon, 0.1% to 1.2% manganese, 0.10% or less sulfur, 0.001% to 0.100% aluminum, 0.10% or less nitrogen, and 0.0020% to 0.020% phosphorus, the balance being iron and incidental impurities, is subjected to hot rolling at a finishing temperature of (Ar₃ transformation temperature−30)° C. or more and a coiling temperature of 400° C. to 750° C., is subjected to pickling and cold rolling, is subjected to continuous annealing including overaging treatment, and is subjected to second cold rolling at a reduction rate of 20% to 50%. 